op berlin



Patented Oct. 8, 1929 UNITED STATES PATENT OFFICE MILTON O. SCHUR, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY, OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS OF REFINING PULP No Drawing.

This invention is directed to the refining of wood pulp to high alpha cellulose content,

using chemicals much cheaper than those heretofore employed for the same purpose and at the same time producing a product of optimum characteristics.

In refining pulp to high alpha cellulose content, it has heretofore been proposed to digest raw pulp in alkaline liquors, and more especially in caustic soda solution which reacts with and dissolvesfrom the pulp nonalpha cellulose components, including beta and gamma celluloses, ligneous matter and resins. The digested pulp is bleachable to high whiteness with a relatively small quantity of bleach, so that minimum degradation of the cellulose into oxycelluloses takes place during bleaching. The bleached product is a suitable substitute for cotton for the manufacture of papers and for the preparation of cellulose derivatives.

Refining chemicals such as caustic soda are quite expensive, so that the production of a high alpha cellulose-containing fiber by this method involves considerable cost, particularly when recovery of the sodium constituent present in the spent liquor of digestion is not made. In patent No. 1,599,489, issued September 14, 1926, to George AQRichter and myself, a refining process is disclosed, in which lime, which is relatively inexpensive, is used in lieu of caustic soda as the refining chemical. As more fully discussed in that patent, if pulp is pretreated with an oxidizing reagent such as chlorine water to effect a reaction upon the ligneous component of the pulp, lime liquor may be subsequently used to effect a refinement of the pulp to a composition comparable to that obtained with caustic soda solution. As stated in the patent, pretreatment of the pulp with chlorine water is essential in order to produce a product having the desired characteristics.

Even when such pretreatment is made, however, it is occasionally experienced that 1f the lime-digested pulp be bleached to substantially pure whiteness, considerable degradation of the alpha cellulose into oxycelluloses results. I have found that the difficulty of bleaching such lime-digested pulp to pure Application fi1ed August 5, 1927. Serial No. 211,008.

whiteness without degrading the alpha cellulose is traceable to the fact that lime digestion does not eflect so complete a removal of resinous impurities, including resins, gums, waxes and fats, as does caustic soda digestion. Thus, whereas caustic soda digestion apparently effects a saponification and solution of the resinous impurities, lime tends to react with these impurities to'produce and fix on the fiber insoluble, highly colored calcium compounds,probably insoluble soaps, resistant to whitening by bleach, especially calcium hypochlorite bleach, which itself tends to oxidize and fix the resinous impurities in the form of oxidized, insoluble products on the fiber. Hence, even with carefully prescribed and skillfully executed processing, it is impossible to attain substantially ultimate or pure whiteness without raising the copper number to an objectionable point.

It is the object of the present invention consistently to produce a substantially pure white fiber of high alpha cellulose content and low copper number, using lime as the refining chemical. In accordance with the present invention, this is accomplished by removing resinous impurities and specks present in the pulp at any time before it is finally bleached, preferably before lime digestion. When such removal is effected, pretreatment of the pulp with an oxidant such as chlorine before lime digestion, while still advantageous in producing a product having optimum characteristics, is not so essential as in the case when resinous impurities and specks are left in the pulp. I

Resinous impurities may be removed from pulp by various methods. One of these methods depends upon the marked difference in amount of these impurities present in dif ferent portions of the pulp. Thus, these impurities are associated largely with the so-called fines, including fibrillae, fiber fragments, and colloidal cellulose. The fines, being more adsorptive than the rest of the pulp, apparently selectively or preferentially adsorb and retain substantially all the resinous impurities to the exclusion of the long and perfect fibers. Hence one of my methods involves the removal of the fines together with their associated impurities. Another method consists in deresinifying and cleansing the pulp by chemical reagents. This may be accomplished by soap-digesting the pulp to effect a dispersion or colloidal solution of the resinous impurities from the pulp, and then washing the digested product, as disclosed and claimed in application, Serial No. 202,454, filed June 29, 1927, by George A. Richter and myself. Another method consists in extracting the resinous impurities by treating the pulp with organic solvents, preferably in counter-current fashion, as described in application, Serial No. 212,000, filed A.u

. gust- 10, 1.927, by Royal H. Rasch and myself.

A specific example of procedure which has been found to be satisfactory in producing a product of the requisite characteristics may be substantially as follows. The pulp employed as a raw material may be derived by digesting chipped wood in a suitable fiberliberating liquor, for example, calcium bisulphite, in accordance with approved modern practice. The liberated pulp is washed and then screened in the usual manner, to effect a removal of coarse impurities. The fines, together with their associated resinous impurities and specks, are then removed from the washed pulp. This may be accomplished by washing the pulp in thin layers on fine-mesh wire screen, say, 50mesh Fourdrinier wire, or on thin plate having fine perforations,for instance, .02 in diameter, as more fully disclosed in application, Serial No. 7 55,7 30, filed December 13, 1924, by George A. Richter and Elwood Ebie. The long or perfect fibers remain on thescreen or plate, while the fines carrying the resinous impurities and specks pass through the openings with the wash water. Washing may also be effected by repeatedly diluting the pulp with fresh filtered water and flowing the pulp over an incline made of wire screen or perforated plate. The washing may be car 'ied out until a removal of from 4% to 10% of material comprising fines containing in some instances as high as 7% matter soluble in carbon tetrachloride is eifected. The quantity of fines and their composition will vary, however, depending upon the severity of the fiber-liberating process and the nature of the wood em ployed as raw material.

In certain cases, it may be more advantageous to effect a separation of the long and perfect fibers from the rest of the pulp, as disclosed in application, Serial No. 206,477, filed July 18, 1927, by Robert A. VVebbe-r. This may be accomplished by passing edgepresenting members capable of picking up long and perfect fibers, through the pulp, and then washing and recovering the picked up pulp from such members.

The perfect pulp thus obtained is now preferably pretreated with 0.5% to l chlorine (or more, if the stock is undercooked),

in the form of chlorine water, until practically all the chlorine has been consumed by reaction, and is then washed. The pretreated pulp is then digested at a density of, say, 10% for five or six ohurs, at about 212 F., in lime cooking liquor containing about 10% active lime, based upon the dry weight of fiber. During digestion, the liquor reacts upon and dissolves beta and gamma celluloses, ligneous matter and other non-alpha cellulose components of the pulp. There is no tendency, however, to produce and fix on the pulp insoluble, highly colored calcium compounds which are resistant to whitening by bleach. After digestion, the pulp is washed clear of the spent liquor and is treated with 1% to 4% bleach (either calcium or so dium hypochlorite), preferably inthe presence of about caustic soda or lime. The bleached product has an alpha cellulose content of about 94%, a copper number of about 1.0 to 1.4, and a color of 110 to 112, this corresponding to substantially pure or ultimate whiteness. These characteristics indicate that the removal of resinous impurities from pulp prior to subjecting it to lime-digestion results in a product of better color and at the same time of even lower copper number than when these resinous impurities are left in the pulp. The product yields nitrocelluloses,

cellulose acetates and other derivatives as free or even freer from coloring matter than the corresponding derivatives prepared from cotton linters or the so-called nitrating grade of rag paper. It is of. excellent color stability, and hence eminently adapted not only for the preparation of high grade stable'derivatives, but also for the manufacture of high grade papers.

While in the specific example cited deresinification of the pulp is accomplished by removing the fines, it is to be understood that any. one or combination of the other deresinifying me hods hereinbefore disclosed may be employed. Furthermore, in lieu of carrying out lime-digestion of the pulp at atmospheric pressure, closed digesters may be used and the temperature of digestion raised above 212 F., but, as previously noted, excellent results are obtainable with less expensive equipment, at 212 F. when the time of digestion is prolonged sufficiently to effect the de sired reaction upon the pulp.

Throughout the specification, lime has been spoken of as the chemical which it is desired to employ as the pulp-refining agent. By the term lime, however. as used in the specification and appended claims, I mean also to include its chemical equivalents, particularly magnesium oxide, or mixtures of both calcium oxide and magnesium oxide, such as would result if dolomite, rather than calcite, constituted the raw material from which the lime was derived.

Although the example cited refers to sulphite pulp, other fibers may be processed, such as pulp liberated with bisulphite liquor containing sodium sulphate, kraft liquor, etc. Excellent results may also be obtained if the pulp is deresinified after the oxidizing pretreatment or after lime digestion, although for economic reasons it is preferred to deresinify the raw pulp. Furthermore, bleach liquor may be used in lieu of chlorine water as the oxidant prior to the lime digestion, or the oxidizing pretreatment may be omitted altogether depending upon local conditions.

In estimating the saving occasioned by the use of lime instead of caustic soda, the cost of the deresinifying treatment and the superior results obtained must, of course, be taken into consideration. The fines removed from the raw pulp in the example cited need not be thrown away, for they are usable as a filler in the manufacture of certain products such as paper sheathing, wallboard, fiber pipe, and the like. Furthermore. if left in the pulp and if caustic is used in the digestion, a material proportion of the fines would be dissolved, substantially decreasing the yield of refined pulp. Finally, deresinified refined pulp is preeminently suited and hence of enhanced value for the production of light colored or colorless cellulose derivatives and of paper of the highest color permanency, for it is free from traces of refractory, unstable colored impurities which survive the usual caustic digestion to an appreciable extent and the presence of which are objectionable in the preparation of cellulose products of the highest quality.

Having thus described certain embodiments of this invention, it should be evident to those skilled in the art that various changes might be made therein without departing from. the spirit or scope of invention as set forth in the appended claims.

What I claim is 1. In a process which includes lime-digesting a pulp, and then bleaching the digested pulp, that step which comprises deresinifying the pulp before such bleaching.

2. In a process which includes pretreating pulp with an oxidant, lime-digesting the treated pulp, and then bleaching the digested pulp, that step which comprises deresinifying the pulp before such bleaching.

8. A process which comprises removing resins from raw pulp, and then digesting the pulp in lime liquor.

1. A process which comprises deresinifying raw pulp, digesting the pulp in lime liquor, and bleaching the digested pulp.

5. A process which comprises deresinifying raw pulp, pretreating the pulp with an oxidant, digesting the pretreated pulp in lime liquor, and bleaching the digested pulp.

6. A process which comprises removing fibrillae, fiber fragments, and colloidal cellulose from raw wood pulp, and digesting the pulp in lime liquor.

7 A process which comprises separating long and perfect fibers from raw pulp, digesting the separated fibers in lime liquor, and bleaching the digested pulp.

8. A process which comprises removing from 41% to 10% ofmaterial comprising fibrillae, fiber fragments, and colloidal cellulose from raw pulp, and digesting the pulp in lime liquor.

9. A process which comprises removing resinous impurities and specks from raw pulp, digesting the pulp in lime liquor, and bleaching the digested pulp in bleach liquor containing free alkali.

10. A process which comprises removing \nost of the resins from raw sulphite pulp, pretreating the pulp with 0.5% to 3% ch1orine, digesting the pretreated pulp with about 10% active lime, washing the pulp, and treating with about 1% to 1% bleach.

11. A process which comprises pretreating raw pulp with an oxidant, deresinifying the pretreated pulp, and digesting the pulp in lime liquor.

12. A process which comprises digesting pulp with lime liquor, deresinifying the digested pulp, and bleaching.

13. A process which comprises pretreating raw pulp with an oxidant, digesting the pre treated pulp in lime liquor, deresinifying the digested pulp, and bleaching.

14. A process which comprises deresinifying raw pulp, digesting the pulp in lime liquor, and bleaching the digestedpulp in calcium hypochlorite liquor.

15. A process which comprises removing fibrillee, fiber fragments, and colloidal cellulose from raw wood pulp, digesting the pulp in lime liquor, and bleaching the digested pulp in calcium hypochlorite liquor.

In testimony whereof I have aifixed my signature.

MILTON O. SCHUR. 

